ALBERT

Description: The prosperity of Neolithic rice agriculture along the Yangtze River Valley (China) under the humid East Asian Monsoon has been well documented. However, the way in which major hydrological changes influenced the expansion of rice farming remains elusive, mainly because detailed climate records associated with critical periods of the development of early rice farming are still lacking. Here we present high-resolution n -alkane carbon ( 13 C) and hydrogen (D) isotope data sampled every 5 cm from a 360 cm sedimentary sequence spanning an ~5 k.y. period at the Tianluoshan archaeological site in China’s eastern coastal region. Combined with micropaleontological records, our dual isotope data reveal a detailed climate change profile between 7.0 and 4.6 ka, showing major hydrological changes that coincided with the early development of rice farming in the lower Yangtze region. Two major evapotranspiration events, indicated by synchronized positive 13 C and D shifts of as much as 5 ( 13 C) and 60 (D), are evident ca. 7.0 ka and 6.4 ka, and coincided with regional sea-level changes. Our new isotope data suggest that these short drought climate conditions superimposed upon local sea-level regressions opened suitable new habitats for expanding rice agriculture in the lower Yangtze Delta when subsequent humid climate regimes returned.

Description: The tectonic evolution of the Lhasa terrane (southern Tibetan Plateau) played a fundamental role in the formation of the Tibetan Plateau. However, many uncertainties remain with regard to the tectonic and paleogeographic evolution of the Lhasa terrane prior to the India-Asia collision. To determine the early tectonic processes that controlled the topographic evolution of the Lhasa terrane, we analyze the Cretaceous strata exposed in the Coqen Basin (northern Lhasa subterrane), which comprises the Langshan and Daxiong Formations. The Langshan Formation unconformably overlies the volcanic rocks of the Lower Cretaceous Zelong Group and consists of ~80 m of Orbitolina -bearing limestones, which were deposited in a low-energy, shallow marine environment. Micropaleontological analysis indicates that the Langshan Formation in the Coqen Basin was deposited from late Aptian to early Cenomanian times (ca. 113–96 Ma). The overlying Daxiong Formation (~1700 m thick) consists of conglomerate, coarse sandstone, and siltstone with interbedded mudstone, and represents deposits of alluvial fans and braided rivers. The Daxiong Formation was deposited after the early Cenomanian (ca. 96 Ma) and accumulated until at least ca. 91 Ma, indicating accumulation rates of greater than 0.3 km m.y. –1 . By combining paleocurrent data, sandstone petrology, detrital zircon U-Pb ages, and Hf isotope analysis, we demonstrate that the Daxiong Formation was derived from Lower Cretaceous volcanic rocks and pre-Cretaceous strata in the northern Lhasa subterrane. During Late Cretaceous time, two thrust systems with opposite vergence were responsible for transforming the northern Lhasa subterrane into an elevated mountain range. This process resulted in the evolution from a shallow marine environment (Langshan Formation) into a terrestrial depositional environment (Daxiong Formation) on the southern margin of the northern Lhasa subterrane. Given the regional paleogeographic context, we conclude that the Daxiong Formation in the Coqen Basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern Lhasa subterrane, which implies early topographic growth of the northern Lhasa subterrane in southern Tibet prior to the India-Asia collision.

Description: Our new stratigraphic, sedimentological, and micropaleontological analysis, integrated with basalt geochemistry, sandstone petrography, and detrital-zircon U-Pb and Hf isotope data, suggests the revision of current models for the geological evolution of the Asian active margin during the Cretaceous. The Xigaze forearc basin began to form in the late Early Cretaceous, south of the Gangdese arc, during the initial subduction of the Neotethyan oceanic lithosphere under the Lhasa terrane. Well-preserved stratigraphic successions document the classical upward-shallowing pattern of the forearc-basin strata and elucidate the origin of the associated oceanic magmatic rocks. The normal mid-ocean-ridge basalt (N-MORB) geochemical signature and stratigraphic contact with the overlying abyssal cherts (Chongdui Formation) indicate that the Xigaze Ophiolite formed by forearc spreading and represents the basement of the forearc sedimentary sequence. Volcaniclastic sedimentation began with thick turbiditic sandstones and interbedded shales in the late Albian–Santonian (Ngamring Formation) followed by shelfal, deltaic, and fluvial strata (Padana Formation), with final filling of the basin by the Campanian age. Forearc sandstones do not show the classical trend from feldspatho-lithic volcaniclastic to quartzo-feldspathic plutoniclastic compositions, indicating limited unroofing of the Gangdese arc prior to collision. U-Pb age spectra of detrital zircons are unimodal with a 107 Ma peak in the lower Ngamring Formation (104–99 Ma), bimodal with a subordinate additional peak at 157 Ma in the middle Ngamring Formation (99–88 Ma), and multimodal with more abundant pre-Mesozoic ages in the upper Ngamring and Padana Formations (88–76 Ma). These three petrofacies with distinct provenances document the progressive erosional evolution of the Gangdese arc, with uplift of the central Lhasa terrane and expanding river catchments to include the central Lhasa terrane during the Late Cretaceous.